[llvm] [SPIRV] Fix printf regression after #178980 (PR #182423)

[Nick Sarnie via llvm-commits]

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@@ -984,6 +993,31 @@ struct SPIRV final : public VariadicABIInfo {
     return {A, false};
   }
 
+  // The SPIR-V backend has special handling for SPIR-V mangled printf
+  // functions.
+  // printf can be mangled in two ways with itanium mangling,
+  // either as a normal printf or with an additional __spirv_ocl_ prefix.
+  // For example:
+  //   _Z6printfPU3AS2Kcz
+  //   _Z18__spirv_ocl_printfPU3AS2Kcz
+  // Ignore those functions if target is SPIR-V
+  bool ignoreFunction(Function *F) override {
+    Module *M = F->getParent();
+    const Triple &T = M->getTargetTriple();
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sarnex wrote:

We really should avoid this kind of target-specific special handling leaking into other passes. Can you provide more information on specifically what is wrong with the IR after `ExpandVariadics` and why it can't be fixed inside the `SPIR-V` backend?

https://github.com/llvm/llvm-project/pull/182423